In recent years, a ZigBee has been used to implement an inexpensive sensor network with lower power consumption using a wireless personal area network (WPAN) scheme. ZigBee is a standard defined from a network layer using IEEE 802.15.4 physical (PHY) and media access control (MAC) layer protocols.
In ZigBee network topology, nodes in a wireless sensor network system are divided into coordinators, routers, and end devices. Here, the coordinator is a top device in a tree structure and manages the tree, and the router is a device functioning as a sub-node of the coordinator or another router and communicates in synchronization with a beacon from the coordinator and the router located on an upper level. In this case, the router may have sub-nodes. The end device is located on the lowest level of the network topology. The end device transmits no beacon, mainly senses an ambient environment using an embedded sensor, and then delivers the sensed data to a router and a coordinator located on an upper level in synchronization with a beacon from the router and the coordinator (a sensor network function), or receives control data from the coordinator and the router to control a controlled subject connected to the end device (a control network function).
In a ZigBee standard, information about neighboring devices is obtained to create a neighbor table and to determine proper beacon transmission and active periods within a range that does not overlap with schedules for the neighboring devices based on the information, in order to prevent a beacon from colliding with other beacons or data transmissions.
However, since the scheduling method in the ZigBee standard is not standardized, problems described below may arise.
FIGS. 1A and 1B are diagrams for explaining problems of a scheduling method in a ZigBee network. In FIGS. 1A and 1B, black periods indicate beacons, an interval between a start point of the black period and a start point of a next black period is a beacon interval (BI), and a combination of the black period and a period of a router R1, and a combination of the black period and a period of a router R2, form super-frame durations SD1 and SD2 of the routers R1 and R2, respectively.
In FIG. 1A, where the two routers R1 and R2 are set to have different communication areas, i.e., sub-networks, there may not be a problem with collision-free scheduling in each sub-network. However, where a new device N is joined to an overlapping communication area between the two routers R1 and R2 as indicated by a dotted line, beacons from the routers R1 and R2 temporally overlap and collide as shown in FIG. 1A. Accordingly, the device N may not correctly receive the beacon.
Meanwhile, where two routers R1 and R2 are initially set to have different communication areas, i.e., sub-networks, as shown in FIG. 1B, and where the router R2 moves to the communication area of the router R1 as indicated by an arrow, or where a communication environment is changed, the beacons from the routers R1 and R2 may collide. Accordingly, a device N1, which is a child node of the router R1, may not receive the beacon, or a data transmission period of a device N2, which is another child node of the router R1, may overlap with the beacon of the router R2 to damage data.